Paraxylene (also “p-xylene” or “PX”) is generally considered the most important of C8 aromatic isomers, being used as an intermediate or starting material for such diverse end uses as synthetic fibers and bottle plastic. Paraxylene is typically obtained from a C8 aromatic hydrocarbon mixture derived from reformate by processes including aromatic extraction and fractional distillation. However, such processes involve high operational costs and result in only limited yields. Increasing selectivity to paraxylene and other operational improvements, especially in the area of separation of paraxylene from other C8 isomers and impurities, is the focus of intense research.
Selectivities to para-xylene in excess of 90 wt % (based on total C8 aromatic product) have been reported by reacting toluene with methanol in the presence of a catalyst comprising a porous crystalline material, preferably a medium-pore zeolite and particularly ZSM-5, having a Diffusion Parameter for 2,2 dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa). See in particular U.S. Pat. Nos. 6,423,879 and 6,504,072.
One problem with these processes has only recently been observed, and that is the production of undesirable oxygenate by-products. Such oxygenate by-products include water, alcohols, ethers, ketones, aldehydes, acids, and phenols. Depending on various factors including the boiling point of the oxygenate, these undesirable by-products are either returned to the alkylation reactor in recycle streams or leave the process through one or more product streams.
In particular, the para-rich xylene product stream (i.e., a product stream having paraxylene in greater than equilibrium amounts, which is approximately 24 mol %, relative to the total xylenes present) tends to contain phenol, methyl phenols and dimethyl phenols. As a result, when the paraxylene is recovered from this product stream, generally by crystallization or by adsorption, the residual para-depleted xylene fraction (i.e., a lower amount of paraxylene than equilibrium amounts, relative to total xylenes) typically contains from ten to several hundred ppmw of phenolic impurities. These impurities limit the value of the para-depleted xylene fraction and generally mean that the fraction can only be used as a blending stream for automotive gasoline.
WO99/38823 teaches a reactive distillation process for the production of xylenes by contacting toluene with a methylating agent. Dimethylether (DME) and unreacted methanol are recycled “to extinction” back to the reactive distillation column. Since it is known that both DME and methanol are useful alkylating agents in this reaction, this solution, at least in hindsight, is not surprising. Reactive distillation systems, however, have numerous drawbacks, particularly in that they are not generally commercially feasible.
One recent improvement in the alkylation of aromatic hydrocarbons with methanol is U.S. Patent Publication 2010-0261941, which is directed to a process using crystallization technology to purify paraxylene from a mixture of large concentrations of C8 aromatics and also small concentrations of oxygenated species.
More recently, a process has been developed whereby the concentration of phenolic impurities in a xylene stream produced by alkylation of benzene and/or toluene with methanol can be reduced to trace levels, e.g., below 0.1 ppmw, by one or more washing treatments with an aqueous solution of a base. The resultant treated xylene stream, if necessary after water washing to remove any phenate-containing solution, can then be recycled to the xylene splitter to generate additional para-xylene or can be used as a solvent. See U.S. patent application Ser. No. 13/487,651.
However, the above solutions do not solve the entirety of problems associated with the presence of oxygenates in the product stream, particularly the presence of phenol. By way of example, there still remains the problem of the presence of oxygenates (other than alkylating agents in the desired reaction) in the unreacted methanol and benzene and/or toluene product stream, as well as water, any of which, if recycled can cause problems in the reactor system, such as catalyst deactivation due to fast coke formation. In addition, there are many other species of oxygenates present in the product stream of the alkylation reactor besides phenolic species. Separation of these oxygenates is costly and difficult. Accordingly, it would be highly beneficial if oxygenates, other than the alkylating agents DME and methanol, could be eliminated or minimized to take better advantage of the para-selectivity of the alkylation of aromatic species with methanol.
The present inventors have surprisingly discovered after careful study that under appropriate conditions such oxygenates will be converted in the reactor without build-up and catalyst performance is not affected by the recycled oxygenates.